In general, light is electromagnetic radiation which oscillates. The frequency of oscillation establishes a wavelength for the light. Light wavelengths in the range of about 4,000 to 7,700 angstroms (Å) are perceived by the normal human eye, with different colors having different wavelengths. Light at other frequencies and wavelengths is not perceived by the human eye.
Polarized light has oscillatory electrical and magnetic properties that are confined to a particular plane. The plane is stationary and extends perpendicular to the path of the light beam. For example, a light beam which propagates in a horizontal beam path may have a vertical plane of polarization. In general, most light visible to the normal human eye is not polarized, meaning that the oscillatory electrical and magnetic properties exist randomly in many different orientations in the plane perpendicular to its propogation.
Polarized light beams are used in many technical and scientific applications, usually under circumstances where multiple light beams occupy the same, common or coincident beam paths. Each light beam can carry separate information encoded into the frequency, wavelength or phase relationship of the polarized light beam. Because polarized light beams have specific planes of polarization, multiple polarized light beams can propagate in the same coincident beam path while still preserving the information unique to each light beam. Each of the light beams remains distinguishable from the other because of its own unique plane of polarization, even though all the beams may occupy the same or a similar beam path.
In technical and scientific applications, it is frequently necessary to separate one polarized light beam from each other individual polarized light beam in a common beam path. Separation is required because it is necessary or desirable to change or modify the information carried by one of the polarized light beams, such as changing its frequency, wavelength or phase relationship without changing its plane of polarization or without influencing the information in the other light beam. Similarly, it is also frequently necessary to add an additional polarized light beam to a common beam path occupied by one or more other polarized light beams. It is important when separating or combining the light beams that the beam which remains in the beam path is not adversely influenced by the separation or combination.
Both the separating and combining requirements are frequently needed when one of the polarized light beams is used to measure or respond to some characteristic before rejoining the common beam path with the other light beam or beams having different planes of polarization. One such exemplary application is described in the above referenced US patent application, where two differently-polarized beams of light are reflected from two separated, freefalling test masses to obtain relative phase and frequency information by which to determine a differential in gravity acting on the two separated, freefalling test masses. After reflection off of the two test masses, the two polarized light beams are compared. The phase and frequency information measures the minute change in separation distance between the two test masses during their free fall. The change in separation distance is directly related to the difference in gravity acting upon the two test masses. This type of instrument is known as a gravity gradiometer.
In many technical and scientific applications, exemplified by the gravity gradiometer described in the above referenced US patent application, there are many and significant practical requirements, all of which must be obtained in order to preserve the functional accuracy and integrity of the application. For example, any change in position of an optical element can adversely affect the information carried by each of the polarized light beams, because the change in position will be reflected in a change in frequency, wavelength and phase of the polarized light beam. Any change in frequency, wavelength and phase of the polarized light beam caused by an unintended external effect adversely changes the information carried by that light beam in an unintended or erroneous manner and thereby appears as an error or inaccuracy. Temperature changes, vibration and external perturbations all have the ability to adversely affect the information carried by the light beams. Temperature changes may cause movement of the optical elements which changes the alignment of the beam paths and thereby diminishes the energy of those beam paths to an extent that makes detection and use inaccurate or impossible. Movement of the optical elements may change the beam path length and thereby change the frequency, wavelength and phase of the polarized light beam in the beam path. There are other types of unintended adverse external physical influences which negatively impact light beams.
There are discrete optical components which may be used for separating one polarized light beam from a common beam path occupied by another polarized light beam, or for combining one polarized light beam with another polarized light beam in a common beam path. However, multiple discrete optical components are typically required for such separation or combination. Each of the separate discrete optical elements consumes a significant amount of space, and the number of the discrete optical elements usually required consumes even more space. These factors make it difficult or impossible to incorporate beam separation and combination optical elements in technical and scientific devices where space is limited.
Another problem is that each of the discrete optical components must be precisely positioned and that position must be maintained independent of the other optical components so that the light beam paths remain oriented and directed to achieve the desired and intended functionality. Changes in temperature, vibration and other external perturbations all have the ability to change the alignment of the discrete optical components. The process necessary to achieve the desired alignment of the discrete optical components is difficult, tedious and time-consuming. Furthermore, maintaining the alignment often depends on external support devices which themselves are subject to changes in temperature, vibration and external perturbations.